1. Field of the Invention
The present invention relates to a laser oscillator having at least one folding mirror between an output coupler and a rear mirror positioned on an optical axis.
2. Description of the Related Art
In a laser oscillator, a mode hopping phenomenon may occur in which an order of an oscillated laser beam is instantaneously changed. When the mode hopping occurs in a laser oscillator used for processing (e.g., cutting of metal), processing defects (e.g., unevenness of a cut surface or roughness of a processed surface) may be generated.
In the prior art, in order to limit the mode hopping, parameters of components constituting the oscillator (such as curvature radiuses of an output coupler and a rear mirror, an inner diameter of a discharge tube, and an opening size, a number and a position of an aperture arranged in the oscillator) are optimized. Further, when it is insufficient to adjust these parameters for limiting the mode hopping, a concave mirror may be used as a folding mirror, while a plane mirror is usually used as the folding mirror. This is because, when the folding mirror is planar, the folding mirror may be expanded by irradiating high power laser bream on the mirror so that the folding mirror may have a convex shape, which may cause the mode hopping.
As a relevant prior art document, JP H02-201981 A discloses a high power laser device using a concave mirror (a concave spherical mirror and a concave cylindrical mirror) as a folding mirror.
A laser used to cut a metal is required to have a high laser output in order to improve cutting performance. It is known that, in an oscillator having a high laser output such as a carbon dioxide laser, a reflective-index distribution of laser gas may be uneven due to thermal distribution of the laser gas during laser oscillation, and a mechanical distortion or strain may occur due to heat deformation of the oscillator. In such a case, the mode hopping can be limited by arranging a directional folding mirror (e.g., a mirror having a toric surface) in the oscillator.
As a relevant prior art document, JP H08-252683 A discloses a laser concentrating device having two concave reflecting mirrors, in which one of the mirrors is a toroidal reflecting mirror, and the other mirror is a spherical, cylindrical or toroidal reflecting mirror.
JP 2011-066300 A discloses a laser resonator having a reflecting mirror, an output coupler for constituting an optical resonator between reflecting surfaces of the reflecting mirror and the output coupler, and a folding mirror positioned between the reflecting mirror and the output coupler so as to deflect a laser beam, in which the folding mirror is a toroidal mirror.
Further, JP 2004-342681 A discloses a laser oscillator intended to improve roundness of a beam mode, by using a reflecting mirror in which inner curvatures in X- and Y-directions are different.
The uneven reflective-index distribution of laser gas and the mechanical distortion or strain, which may cause the mode hopping, may occur by heat generated during the laser oscillation. As a result, an amount of heat generated during the laser oscillation is varied depending on the condition of the oscillator. Further, the amount of generated heat is significantly different between immediately after the laser oscillator is assembled or manufactured and after the laser oscillator is used over a long duration, due to a dust, etc., adhered to a mirror within the oscillator, etc. Therefore, the reflective-index distribution and the mechanical distortion or strain may be different between immediately after the oscillator is manufactured and after the oscillator is used over a long duration. In other words, even when a directionality of a reflecting surface at the time of manufacture is appropriate with respect to the reflective-index distribution and the direction of the mechanical distortion or strain, the directionality of the reflecting surface may become inappropriate after being used over a long duration. In such a case, the mode hopping cannot be reduced in the configuration of JP H08-252683 A or JP 2011-066300 A, since the direction of the folding mirror cannot be changed.
In the configuration of JP 2004-342681 A, each of the outcoupling mirror and the rear mirror has the reflecting mirror having in which inner curvatures in X- and Y-directions are different. Due to such a configuration, an oscillated beam mode may have an ellipse shape. A high power laser oscillator has a plurality of discharge areas for obtaining a high laser output, and the directionality of the reflective-index distribution may be different in each discharge area. JP 2004-342681 A does not disclose a means for optimizing the beam mode in each discharge area in such a case.
In addition, in JP H02-201981 A, a mirror having a cylindrical shape is used as a folding mirror. However, the cylindrical mirror is always combined with a spherical mirror, and the curvature radius and the direction of the cylindrical mirror are limited in order to reduce or eliminate an aberration of the spherical mirror. The (occurrence) frequency of the mode hopping is not affected by the direction of the cylindrical shape for reducing or eliminating the aberration, and thus it may be difficult to limit the mode hopping by the combination of the cylindrical mirror and the spherical mirror.